1. Field of the Invention
The present invention relates generally to laser treatment of tissue, and more particularly to photoselective vaporization of tissue, including uterine tissue, as applied to the treatment of gynecological conditions.
2. Description of Related Art
A commonly employed procedure for removal of tissue in the treatment of gynecological conditions involves the use of a hysteroscope and a small wire loop energized by radio frequency energy to cut tissue.
Nd:YAG lasers delivering output with a wavelength of 1064 nm have been used for the treatment of gynecological conditions such as the ablation of the endometrium. Although 1064 nm light is hemostatic at high power levels, its low absorption in blood and uterine tissue leads to inefficient ablation and a large residual layer of thermally denatured tissue several millimeters thick.
High power densities are required for rapid and efficient vaporization of tissue. The difficulty of achieving higher average output power densities is that when high input powers are supplied to the laser element from an excitation source such as an arclamp a large amount of heat is generated in the lasing element. This heat induces various deleterious effects in the lasing element. In particular the temperature difference between the coolant and the hot lasing element generates a thermally induced graded index lens that decreases the beam quality of the laser and causes the laser to operate with more transverse optical modes than it would otherwise.
The M2 parameter is a well established convention for defining the beam quality of a laser and is discussed in pages 480-482 of Orazio Svelto and David C. Hanna, Principles of Lasers, Plenum Press, New York, 1998, which is incorporated herein by reference. The beam quality measures the degree to which the intensity distribution is Guassian. The quantity M2 is sometimes called inverse beam quality rather than beam quality but in this application it will be referred to as beam quality. M2 is defined as                     M        x        2            ≡                                    (                                          σ                x                            ⁢                              σ                f                                      )                    NG                                      (                                          σ                x                            ⁢                              σ                f                                      )                    G                      =          4      ⁢                           ⁢                        π          ⁡                      (                                          σ                x                            ⁢                              σ                f                                      )                          NG              ,where π refers to the number 3.14 . . . , σ is used to represent the spot size, the subscripts x and f represent the spatial and frequency domains along the x-axis, respectively, and the subscripts G and NG signify Guassian and non-Guassian, respectively. The x-axis is transverse to the direction of propagation of the beam. The beam quality in any direction transverse to the beam may be essentially the same. Therefore the subscript x is dropped from the M2 elsewhere in the specification. The beam widths or σs are determined based on the standard deviation of the position, where the squared deviation of each position is weighted by the intensity at that point. The beam width in the frequency domain σf is the beam width of the beam after being Fourier transformed.
The formula usually used for calculating the angular divergence, θ, of a beam of light of wavelength λ is strictly valid only for a beam having a Guassian intensity distribution. The concept of beam quality facilitates the derivation of the angular divergence, θ, for the beam with a non-Guassian intensity distribution, according to   θ  =                    M        2            ⁡              (                              2            ⁢                                                   ⁢            λ                                π            ⁢                                                   ⁢                          σ              x                                      )              .  
For example, a TEM00 laser beam has a high beam quality with an M2 of 1, whereas by comparison, high power surgical lasers operate with M2 values greater than 100.
The Applicants have recognized that high power lasers typically have an M2>144. The larger number of modes makes M2 larger and makes it difficult to focus the light into small, low numerical aperture fibers and reduces the ability to project high power density light onto tissue. As a result, the vaporization efficiency of CW arclamp pumped 532 nm lasers is significantly reduced.
Surgical procedures within the uterus have unique risks. For example, precision surgery is of high importance for patients who want to maintain their fertility. Any surgery in the uterus must avoid weakening of the wall of the uterus, which could lead to complications during pregnancy. Also, the physiological diversity of the uterus increases the difficulty of intrauterine operations. The cornual areas of the uterus represent a vulnerable portion of the uterus. In case of a myoma in the cornu, the uterine wall is further thinned by the myoma, which increases the risk of intraoperative perforation of the uterine wall. Even if perforation does not occur, the presence of a thin uterine wall could predispose the patient to bowel injury. As stated by Indman, J. Reproduct. Med. 1991, lack of precise knowledge of the minimum thickness of the uterine wall may be the limiting factor in determining the safety of use of the 1064 nm Nd:YAG laser for endometrial ablation.